Upon activation Dendritic Cells (DCs) become professional Antigen Presenting Cells, able to stimulate naive and memory T cells and initiate an immune response. How long DCs can survive after activation and efficiently present Ags to T and B lymphocytes is still unclear. Successful regulation of the immune response depends on the capacity of DCs to survive for a period of time long enough to stimulate a sufficient number of Ag-specific T cells, and to cease presenting when the response is no longer necessary, for example when the pathogen has been cleared. It is the goal of this proposal to study the survival of dendritic cells and the mechanisms involved in their death after interaction with responding T cells, and their resistance to the effects of such interactions depending on their state of differentiation and activation. The goals of the project will be approached in vitro and in vivo in experimental mice. In vitro the effects of resting and activated T cells on the viability of cultured dendritic cells will be determined, using different apoptotic assays to study the time of death of dendritic cells after co-incubation with T cells. In vivo the fate of traceable dendritic cells, injected into mice in a manner resembling the protocols that use DCs as vaccines, will be followed by immunohistochemistry and flow cytometry studies, in the draining lymph nodes, in the absence or the presence of primary or secondary immune responses. Studies in vitro and in vivo of the MHC-Ag-restriction of the T cell effects on DCs, will reveal whether an immune response can influence bystander dendritic cells. The comparison of survival of DCs grown from wild type or lpr autoimmune mice, which are deficient in the function of the Fas cell-death receptor, will determine the requirement for this death receptor in the regulation of the life span of DCs during an immune response. Testing DCs both in vitro and in vivo in different states of differentiation and stimulated by different activators, may help to find DCs that are resistant to the effects of T cells. Micro-Array analysis and extensive molecular studies of so treated DCs will help to identify the factors involved in their survival/death. These candidate factors will be tested to create new protocols to obtain DCs with a longer life span and/or that are resistant to T cell-induced apoptosis. The results of this project may have a strong practical impact in vaccine development. DCs have been used as vaccine carriers to improve immunity (e.g. immunotherapy of tumors). The identification of factors that reduce or prolong DC survival can be used to increase the efficacy of traditional adjuvants as well as protocols that use DCs as vaccines. It may give us new clues into the pathogenesis of immunopathologies, such as autoimmune diseases, where this regulation may be altered. The experimental evidence obtained from this project will provide the foundation for future investigations concerning the role of DCs in the pathogenesis of autoimmune diseases, to test the hypothesis that DCs may induce/maintain autoimmunity because of deficient or delayed apoptosis. The clarification of the role of DCs in autoimmunity will help to understand which is the right cell to target in the therapy of autoimmune diseases.